The present invention relates to safety devices in general, and more particularly to improvements in safety devices which can be used in fluid-operated machines, such as blow molding machines for the production of bottles or analogous hollow shaped articles. Still more particularly, the invention relates to improvements in means for controlling the movements of mobile screens, shrouds, shields or analogous devices which are used in or with various types of machines to shield the moving parts of such machines and to thereby protect the attendants whenever the fluid-operated motor or motors of the machines are in operation or ready for operation.
It is already known to provide a fluid-operated machine with a safety device which consists of or includes a mobile screen, shield or shroud (hereinafter referred to as shield for short) and which prevents access to moving parts of the machine whenever the means for transmitting motion to such parts is in operation or ready for operation. For example, if the means for moving certain parts of the machine comprises a hydraulic motor receiving pressurized fluid by way of a valve which is installed in a pressure line connecting a pump or another source of pressurized fluid with the motor, a connection between the motor and the source of pressurized fluid can be established only when the shield is held in the operative position, i.e., when the shield prevents access to the moving part or parts of the machine. Even partial shifting of the screen from its operative position entails automatic interruption of the operative connection between the source and the motor. Examples of machines which must be provided with such screens are blow molding machines which are used for the production of large containers (e.g., containers having a capacity of up to and in excess of 200 liters). The sections of the blow mold are movable between open and closed positions by hydraulic cylinder and piston units which generate very large forces, i.e., forces of a magnitude which suffices to cause serious injury to or death of a careless or unskilled attendant. It is often necessary to gain access to the cavities of the mold sections prior to introduction of a parison which is converted into a hollow shaped article, for example, in order to introduce threaded nipples or like inserts which are integrated into the material of the blow molded articles. In many instances, the inserts are introduced by hand so that the protective shield must be moved from its operative position before the mold sections are caused to close around a parison. The introduction of inserts is but one of many reasons that an attendant must gain access to the mold sections when the blow mold is open; for example, it is often necessary to inspect the mold sections and/or other components which are accessible only when the blow mold is open. Still further, a careless attendant is often tempted to assist the introduction of a parison into the space between the open sections of a blow mold, a practice which can lead to serious injury, maiming or death. Therefore, the provision of a protective shield which prevents access to (i.e., which shields) the moving parts of such machines is an absolute necessity which is enforced by authorities in many countries. The arrangement must be such that the fluid-operated motor or motors of the machine are automatically disconnected from the source or sources of pressurized fluid whenever the shield does not assume its operative position. It is further necessary to insure that the fluid-operated motor or motors cannot be actuated when the sections of a blow mold must be replaced or exchanged, i.e., when the attendant or attendants must enter or place their hands into the space between the platens for the mold sections.
In certain presently known machines of the above outlined character, the means for insuring that the shield is held in operative position when the motor or motors are ready for operation includes two limit switches. One of the limit switches completes the circuit of a solenoid-operated valve in the hydraulic connection between the motor or motors and the source of pressurized fluid when the shield assumes its operative position so that the valve admits or can admit fluid to the motor or motors. The other limit switch opens the circuit of the valve so that the valve closes as soon as the shield leaves its operative position. Such safety devices are not reliable because the limit switches establish or terminate a connection between the source of pressurized fluid and the motor or motors through the intermediary of a further part (such as the aforementioned solenoid-operated valve) which is likely to jam and to thereby allow pressurized fluid to reach the motor or motors when the shield is not held in the operative position. Moreover, a careless attendant is likely to intentionally deactivate the other limit switch so that the shield remains in open position while the valve is free to admit pressurized fluid to the motor or motors.
In accordance with another earlier proposal, the shield is provided with a trip (e.g., a wedge-like cam) which actuates a pilot valve as soon as the shield leaves its operative position. The pilot valve thereby causes a shutoff valve to seal the source of pressurized fluid from the motor or motors for the moving part or parts of the machine. As a rule, the shutoff valve has an annular seat for the valve element. Thus, even minor impurities which deposit on the seat and/or on the valve element can prevent complete closing of the shutoff valve so that the latter continues to admit pressurized fluid to the motor or motors after the shield leaves its operative position. Another drawback of such systems is that, if the shield is to actuate the pilot valve immediately after it leaves the operative position, the slope of the aforementioned wedgelike trip or cam must be very pronounced in order to make sure that a relatively minor displacement of the shield from the operative position invariably suffices to actuate the pilot valve and to result in immediate closing of the shutoff valve. Such actuation of the pilot valve by a steep trip or cam necessitates the exertion of a very pronounced force in order to move the shield from its operative position. This is tiresome of the attendants if the shield must be opened by hand because a relatively large force must be applied immediately after the shield is set in motion or simultaneously with initial stage of movement of the shield from its operative position. If the shield is moved by a motor or another suitable prime mover, the energy requirements of the prime mover are very high. The problem is especially acute when the shield must be moved back and forth at frequent intervals, e.g., after each opening of a blow mold.